An adequate supply of oxygen to tissues is essential in maintaining mammalian cell function and physiology. A deficiency in oxygen supply to tissues is a characteristic of a number of pathophysiologic conditions in which there is insufficient blood flow to provide adequate oxygenation, for example, ischemic disorders, cancer, and atherosclerosis. The hypoxic (low oxygen) environment of tissues activates a signaling cascade that drives the induction or repression of the transcription of a multitude of genes implicated in events such as angiogenesis (neo-vascularization), glucose metabolism, and cell survival or death. A key to this hypoxic transcriptional response lies in the transcription factor, the hypoxia-inducible factors (HIF). HIFs are overexpressed in a vast array of cancers through hypoxia-dependent and independent mechanisms, and expression is associated with poor patient prognosis.
HIFs consist of an oxygen-sensitive HIFα subunit and constitutively expressed HIFβ subunit. When HIFs are activated, the HIFα and HIFβ subunits assemble a functional heterodimer (the α subunit heterodimerizes with the β subunit). Both HIFα and HIFβ have two identical structural characteristics, a basic helix-loop-helix (bHLH) and PAS domains (PAS is an acronym referring to the first proteins, PER, ARNT, SIM, in which this motif was identified). There are three human HIFα subunits (HIF-1α, HIF-2α, and HIF-3α) that are oxygen sensitive. Among the three subunits, HIF-1α is the most ubiquitously expressed and induced by low oxygen concentrations in many cell types. HIF-2α is highly similar to HIF-1α in both structure and function, but exhibits more restricted tissue-specific expression, and might also be differentially regulated by nuclear translocation. HIF-3α also exhibits conservation with HIF-1α and HIF-2α in the HLH and PAS domains. HIFβ (also referred to as ARNT—Aryl Hydrocarbon Receptor Nuclear Translocator), the dimerization partner of the HIFα subunits, is constitutively expressed in all cell types and is not regulated by oxygen concentration.